Type I mucopolysaccharidosis (MPS), also known as Hurler's syndrome, is an inherited metabolic disease caused by a defect in the enzyme α-L-iduronidase (IDUA), which functions to degrade mucopolysaccharides. An insufficient level of IDUA causes a pathological buildup of heparan sulfate and dermatan sulfate in, e.g., the heart, liver, and central nervous system. Symptoms including neurodegeneration and mental retardation appear during childhood and early death can occur due to organ damage. Typically, treatment includes intravenous enzyme replacement therapy with recombinant IDUA. However, systemically administered recombinant IDUA does not cross the blood brain barrier (BBB), and therefore has little impact on the effects of the disease in the central nervous system (CNS).